As CMOS transistors continue to be scaled down in size with each new generation, the sub-threshold leakage power (also referred to as the stand-by power) in these transistors becomes an increasing fraction of the total power dissipation. This has lead researchers to explore potential alternatives to CMOS technology. NEM switching devices, when used as a replacement for CMOS transistors or used in combination with CMOS transistors promise to substantially reduce or eliminate the sub-threshold leakage power which is present with CMOS transistors. NEM switching devices also promise advantages in terms of increased radiation hardness and higher temperature operation as compared to CMOS transistors.
In NEM switching devices, a flow of electrical current between source and drain electrodes is controlled electrically and mechanically using a cantilever arm which moves in response to an applied gate voltage to make or break an electrical connection between the source and drain electrodes. Removing the gate voltage from the NEM switching device interrupts the electrical connection between the source and drain electrodes, thereby resulting in an extremely low leakage current which is due to Brownian motion of the cantilever arm and tunneling.
Over the past decade, numerous designs for NEM switching devices have been proposed (see e.g. U.S. Pat. Nos. 5,638,946; 6,548,841; 7,256,063 and 7,355,258) which have been primarily based on the use of cantilever arms which are electrostatically actuated to move into contact with an underlying drain electrode. Carbon nanotubes are also being explored for use in NEM switching devices (see e.g. U.S. Pat. No. 7,256,063).
The present invention provides a NEM switching device in which the source electrode comprises an electrically-conductive beam which is suspended above a substrate and anchored at each end thereof to the substrate. This arrangement provides a higher restoring force and a higher resonant frequency than devices which utilize cantilever arms, thereby promising faster turn-on and turn-off times, and also being less susceptible to stiction (i.e. adhesion of the beam to a drain electrode). This arrangement also avoids problems of curl-up of cantilever arms due to internal stress, and stiction of the cantilever arms to the drain electrode upon release of the cantilever arms.
The NEM switching device of the present invention utilizes an electrically-conductive beam which can be formed of ruthenium metal with a ruthenium oxide coating thereon. The ruthenium oxide coating, which can be optionally provided on the NEM switching device of the present invention, is useful to prevent carbon fouling of contacting surfaces of the NEM switch to improve device reliability.
The NEM switching device of the present invention can also be formed with gate and drain electrodes located on both sides of the electrically-conductive beam. This can be advantageous to provide a single pole double throw (SPDT) switching of the device. This can also be advantageous when forming logic circuits from a plurality of NEM switching devices. In such logic circuits, logic state inputs can be provided to the gate electrodes on one side of the electrically-conductive beam while the gate electrodes on the other side of the electrically-conductive beam can be used as disable inputs to disable that logic circuit, as needed, and to provide an output which is either electrically floating (i.e. electrically disconnected from the inputs or any power supply voltage) or tied to a power supply voltage (either Vdd or Vss). The use of disable inputs to the logic circuit when it is in a stand-by mode reduces the power consumption for that logic circuit and for any other logic circuits which are connected to it. These disable inputs also provide an ability to reconfigure a logic circuit by removing one or more NEM switching devices from the logic function.
The NEM switching device of the present invention can also be used to form programmable logic circuits which can be programmed, as needed, to change from being NAND gates to being NOR gates.
Additionally, the NEM switching device of the present invention can be used to form memory cells including SRAM and DRAM memory cells.
SRAM memory cells can be formed according to the present invention with a cross-coupled data latch which requires only two NEM switching devices as compared to the normal four transistors in conventional CMOS SRAM memory cells. The SRAM memory cells formed using the NEM switching devices of the present invention will also have a near-zero stand-by power consumption due to a push-pull operation resulting from the gates and drains located on opposite sides of the electrically-conductive beam and also due to the extremely low leakage current of the NEM switching devices.
DRAM memory cell arrays can be formed according to the present invention which require only one-half the number of NEM switching devices compared to conventional CMOS DRAM memory cell arrays since each NEM switching device is capable of addressing two storage capacitors. Each DRAM memory cell formed according to the present invention will also have a much lower leakage current compared to a conventional CMOS DRAM memory cell; and this will allow a longer charge retention time.
These and other advantages of the present invention will become evident to those skilled in the art.